Energetically expensive oxygen reduction reactions (ORR) at the cathode have been the rate determining step and hence, a severe hindrance to efficient and clean electrochemical energy conversions in low-temperature proton exchange membrane fuel cells (PEMFCs) [1-7]. To promote the ORR activities, Pt based nanocatalysts have largely been used for most commercial applications [6, 8-10]. Yet, the cost of precious metal based catalysts added to the lack of stability and durability of Pt under the highly corrosive and acidic conditions of fuel cell operations have prompted a large volume of research in recent years geared towards the development of transition metal based alloys and/or, intermetallic materials with low Pt-loading [11-16]. Specifically, recent U.S. DRIVE fuel cell technical roadmap has established the 2020 target for the total loading of Pt group metals (PGM) to be approximately 0.125 mg/cm2 electrode area for PEMFC electrocatalysts [17]. To this end, alloyed nanocatalysts have gained tremendous research interest in the past decade due to their unique geometric and/or electronic characteristics that dramatically enhance their catalytic activities, while reducing the net PGM content [18-24]. Alloying Pt with transition metals such as Co, Ni, Cu, etc., have been found to effectively shrink the lattice constant (geometric effect) and tune the d-band center (electronic effect), resulting in a moderate oxygen binding energy (eV) and consequently improved specific and mass activities for electrocatalytic ORR processes [3, 19, 20, 25-28].
Among the aforementioned Pt based nanoalloys (NAs), PtCo systems have attracted the most attention due to its relatively higher activity and stability for the ORR process [5, 29-36]. The nominal Pt:Co ratio as well as the degree of alloying in these nanocatalysts play a critical role in tuning the nanoscale crystalline structures and band structures which in turn dictate the aforementioned geometric and electronic effects responsible for tailoring their ORR catalytic activities [4, 37, 38, 30]. Conventional PtCo alloys were usually prepared by simultaneous reduction of cobalt salts (e.g., Co(NO3)2, CoCl2) and platinum precursors (Pt(acac)2, K2PtCl4, H2PtCl4) in either organic or aqueous conditions, and almost always involve the use of external and indispensible stabilizing agents (CTAB, PVP, oleylamine, etc.) [1, 37, 34]. Recently, a wide range of synthesis techniques have been developed that include impregnation [30], solvothermal method [39], tandem decomposition and chemical reduction [22], polyol method [40], reverse micelle method [41], replacement reaction [42], etc. Yet, most of those synthesis techniques involve wet chemical routes that require intricate steps and even these techniques inevitably use harsh unwanted chemicals in the form of surfactants and/or, stabilizing agents. These organic residues on the nanoparticle (NP) surface are detrimental to their interfacial catalytic properties and eventually, systematic removal of those organic encapsulations from these alloyed and/or intermetallic NPs becomes a challenging and critical step in itself for large-scale production of nanocatalysts. Besides, a fine control of the Pt:Co atomic ratios and alloying degrees for systematic synthesis of a wide range of nanocatalysts still remains elusive in most of these techniques, thereby restricting the application of these ORR catalysts to only limited environmental conditions [5, 41, 38, 40, 43].
Additionally, a few recent attempts have synthesized designer nanocomposites (NCs) made from the best of both ORR (e.g., Pt NPs) and oxygen evolution reaction (OER) catalysts (e.g., transition metal oxides) However, clean synthesis of these complex nanocatalysts in a facile, cheap, and reproducible manner still remains elusive. Even here, most synthesis techniques for metal and metal oxide NPs involve wet chemical routes that require intricate experimental steps involving indispensable chemicals such as surfactants, organic ligands, reducing agents, etc. that block their active surface catalytic sites. [30, 63, 64, 65] Many metal/metal oxide NCs made from perkovsite based oxides are complicated to synthesize and require multi-step processes with harsh chemical conditions and residues. [66, 67, 68] Finally, removal of organic encapsulation (ligands and/or surfactants) from metal/metal oxide NPs itself is a challenging and critical step in their preparation. [69]
As a consequence, compositions, systems, and methods for producing NAs and/or NCs that allow precise construction of inter-atomic structures and extent of alloying in facile, cheap, and reproducible manners are needed.